The present invention relates to a method and system for aligning an ultraviolet beam and an optical fiber preparatory to the manufacture of an optical filter in the optical fiber.
Optical filters comprising Bragg gratings formed in optical fibers are useful for dispersion compensation in optical communication systems, and for various other applications. A known method of manufacturing such filters illuminates a photosensitive optical fiber with ultraviolet light through a phase mask, thereby imprinting a modulation pattern in the refractive index of the fiber core. To obtain uniform modulation, the manufacturing process preferably uses a highly stable ultraviolet light source, such as a continuous-wave argon ion laser apparatus emitting an ultraviolet beam by second harmonic generation.
The beam emitted by this type of light source is relatively weak. To increase the beam intensity, the beam is focused to a width comparable to the diameter of the optical fiber, and scanned lengthwise along the fiber. The fiber diameter is only about one hundred twenty-five micrometers (125 .mu.m), so successful scanning requires careful alignment between the beam and the fiber. In particular, the fiber must be accurately aligned at the beginning of the scan, so that the fiber is centered under the beam and the fiber axis is aligned in the scanning direction.
During the scanning process, alignment can be maintained by the known method of monitoring visible light produced by fluorescence when the core of the optical fiber is illuminated by the ultraviolet beam. Some of this visible light propagates through the fiber core, and can be measured by an optical power meter connected to one end of the optical fiber. The measured value can be fed back to an automatic positioning system that keeps the beam centered on the fiber core.
It would be advantageous if the initial alignment of the fiber could also be carried out by automatic control.